Increased the glyph atlas resolution 2x (#162555)

issue: https://github.com/flutter/flutter/issues/149652

This makes https://github.com/flutter/flutter/issues/149652 better. It
doesn't completely it better though, there is still a shrinking of
whitespace at larger scales. Without this patch though the test will
fail with many 3 pixel jumps between glyphs.

I think scaling in the thousands is reasonable so this is an adequate
test.

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/impeller/display_list/aiks_dl_text_unittests.cc

@@ -31,6 +31,7 @@ struct TextRenderOptions {
   DlColor color = DlColor::kYellow();
   DlPoint position = DlPoint(100, 200);
   std::shared_ptr<DlMaskFilter> filter;
+  bool is_subpixel = false;
 };
 
 bool RenderTextInCanvasSkia(const std::shared_ptr<Context>& context,
@@ -57,6 +58,9 @@ bool RenderTextInCanvasSkia(const std::shared_ptr<Context>& context,
   }
   sk_sp<SkFontMgr> font_mgr = txt::GetDefaultFontManager();
   SkFont sk_font(font_mgr->makeFromData(mapping), options.font_size);
+  if (options.is_subpixel) {
+    sk_font.setSubpixel(true);
+  }
   auto blob = SkTextBlob::MakeFromString(text.c_str(), sk_font);
   if (!blob) {
     return false;
@@ -153,10 +157,12 @@ TEST_P(AiksTest, CanRenderTextFrameWithHalfScaling) {
 
 TEST_P(AiksTest, CanRenderTextFrameWithFractionScaling) {
   Scalar fine_scale = 0.f;
+  bool is_subpixel = false;
   auto callback = [&]() -> sk_sp<DisplayList> {
     if (AiksTest::ImGuiBegin("Controls", nullptr,
                              ImGuiWindowFlags_AlwaysAutoResize)) {
       ImGui::SliderFloat("Fine Scale", &fine_scale, -1, 1);
+      ImGui::Checkbox("subpixel", &is_subpixel);
       ImGui::End();
     }
 
@@ -168,7 +174,8 @@ TEST_P(AiksTest, CanRenderTextFrameWithFractionScaling) {
     builder.Scale(scale, scale);
     RenderTextInCanvasSkia(GetContext(), builder,
                            "the quick brown fox jumped over the lazy dog!.?",
-                           "Roboto-Regular.ttf");
+                           "Roboto-Regular.ttf",
+                           TextRenderOptions{.is_subpixel = is_subpixel});
     return builder.Build();
   };
 

engine/src/flutter/impeller/display_list/dl_golden_unittests.cc

@@ -366,6 +366,22 @@ int32_t CalculateMaxY(const impeller::testing::Screenshot* img) {
   }
   return max_y;
 }
+
+int32_t CalculateSpaceBetweenUI(const impeller::testing::Screenshot* img) {
+  const uint32_t* ptr = reinterpret_cast<const uint32_t*>(img->GetBytes());
+  ptr += img->GetWidth() * static_cast<int32_t>(img->GetHeight() / 2.0);
+  std::vector<size_t> boundaries;
+  uint32_t value = *ptr++;
+  for (size_t i = 1; i < img->GetWidth(); ++i) {
+    if (((*ptr & 0x00ffffff) != 0) != ((value & 0x00ffffff) != 0)) {
+      boundaries.push_back(i);
+    }
+    value = *ptr++;
+  }
+
+  assert(boundaries.size() == 6);
+  return boundaries[4] - boundaries[3];
+}
 }  // namespace
 
 TEST_P(DlGoldenTest, BaselineHE) {
@@ -399,5 +415,42 @@ TEST_P(DlGoldenTest, BaselineHE) {
   int32_t y_diff = std::abs(left_max_y - right_max_y);
   EXPECT_TRUE(y_diff <= 2) << "y diff: " << y_diff;
 }
+
+TEST_P(DlGoldenTest, MaintainsSpace) {
+  SetWindowSize(impeller::ISize(1024, 200));
+  impeller::Scalar font_size = 300;
+  auto callback = [&](const char* text,
+                      impeller::Scalar scale) -> sk_sp<DisplayList> {
+    DisplayListBuilder builder;
+    DlPaint paint;
+    paint.setColor(DlColor::ARGB(1, 0, 0, 0));
+    builder.DrawPaint(paint);
+    builder.Scale(scale, scale);
+    RenderTextInCanvasSkia(&builder, text, "Roboto-Regular.ttf",
+                           DlPoint::MakeXY(10, 300),
+                           TextRenderOptions{
+                               .font_size = font_size,
+                           });
+    return builder.Build();
+  };
+
+  std::optional<int32_t> last_space;
+  for (int i = 0; i <= 100; ++i) {
+    Scalar scale = 0.440 + i / 1000.0;
+    std::unique_ptr<impeller::testing::Screenshot> right =
+        MakeScreenshot(callback("ui", scale));
+    if (!right) {
+      GTEST_SKIP() << "making screenshots not supported.";
+    }
+
+    int32_t space = CalculateSpaceBetweenUI(right.get());
+    if (last_space.has_value()) {
+      int32_t diff = abs(space - *last_space);
+      EXPECT_TRUE(diff <= 1)
+          << "i:" << i << " space:" << space << " last_space:" << *last_space;
+    }
+    last_space = space;
+  }
+}
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/impeller/entity/contents/text_contents.cc

@@ -183,7 +183,6 @@ void TextContents::ComputeVertexData(
       Point screen_glyph_position =
           (screen_offset + unrounded_glyph_position + subpixel_adjustment)
               .Floor();
-
       for (const Point& point : unit_points) {
         Point position;
         if (is_translation_scale) {

engine/src/flutter/impeller/typographer/text_frame.cc

@@ -56,7 +56,7 @@ Scalar TextFrame::RoundScaledFontSize(Scalar scale) {
   // CTM, a glyph will fit in the atlas. If we clamp significantly, this may
   // reduce fidelity but is preferable to the alternative of failing to render.
   constexpr Scalar kMaximumTextScale = 48;
-  Scalar result = std::round(scale * 100) / 100;
+  Scalar result = std::round(scale * 200) / 200;
   return std::clamp(result, 0.0f, kMaximumTextScale);
 }